GB2063485A

GB2063485A - Contactless Detection of the Distance of a Metal Surface from a Counter-surface

Info

Publication number: GB2063485A
Application number: GB8036635A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1979-11-15
Filing date: 1980-11-14
Publication date: 1981-06-03
Also published as: IT8025900A0; IT1134212B; FR2469698A1; JPS5684501A; DE2946062A1

Abstract

A spirally convoluted coil (10) is applied to a carrier (1) or substrate and lies opposite a central portion (8) of a metallic diaphragm of a pressure sensor. A square wave voltage (21) is supplied to the coil via a resistor and the resulting voltage peaks (U1) across the coil are rectified and filtered to provide an output voltage (Ua) whose magnitude depends upon the distance between the coil (10) and the diaphragm and thereby upon the pressure applied to the diaphragm. A further spiral coil may be applied to the other surface of carrier (1) where it faces an additional metallic diaphragm. <IMAGE>

Description

SPECIFICATION Measuring Device for the Contactless Detection of the Distance of a Metal Surface from a Ccunter-surface The invention relates to a measuring device for the contactless detection of the distance of a metal surface from a counter-surface, such as the distance between a resilient diaphragm of a pressure sensor and the sensor housing carrying the diaphragm.

The known, contactless methods of measuring the distance between a sensor and a metal body include the eddy current measuring method. An object of the invention is to provide a measuring device which is based on the eddy current measuring method and which can be used economically in mass-produced products, particularly in pressure sensors which are usable in motor vehicles.

Accordingly the invention resides in a measuring device for the contactless detection of the distance between a metal surface and a counter-surface, in which a spiral electrical coil is disposed on a counter-surface which faces the said metal surface.

Advantageously, the preferably planar countersurface is on a non-metallic carrier which is preferably made from insulating material. In a further development of the invention, the coil is applied to the carrier by vapour deposition or by printing. In a further development of the invention, the measuring device is provided with an evaluation circuit in which a generator for generating an oscillator voltage of 10 KHz to approximately 10 MHz, preferably of approximately 200 KHz, is connected to the coil via a series resistor, and means for detecting the residual voltage which appears on the coil, and which varies in dependence upon the distance to be detected, is connected in parallel with the coil.

In a further development of the invention, the evaluation circuit includes a rectifier, such as a diode, which is connected to the core, and there is connected in series with this diode or with this rectifier a filter network which operates as a lowpass filter and which comprises at least one storage capacitor disposed in a shunt arm, a discharge resistor connected in parallel with the storage capacitor, a series resistor connected in series with the rectifier, and a shunt capacitor connected in the series resistor.

In order to form strong eddy currents in the surface whose distance from the coil varies, it is particularly advantageous if the generator supplies a square-wave voltage. Advantageously, for this purpose, the generator can be in the form of an astable multivibrator.

The invention will be further described by way of example, with reference to the drawings, in which: Fig. 1 is a somewhat diagrammatic crosssectional view of a pressure sensor operating in accordance with the eddy current method; Fig. 2 shows an evaluation circuit; Fig. 3 shows several time graphs for illustrating the voltages appearing at the individual stages of the evaluation circuit, and Figs. 4 to 6 show further advantageous embodiments of pressure sensor.

The pressure sensor of Figure 1 has a carrier 1 which is made from ceramic material Awl203 and onto the planar top side 2 of which is applied a highly adhesive, solderable coating 3. A hardened metal plate 4, which is actually only approximately 1.5 mm thick, is mounted on the coating 3 and its continuous edge zone 5 seated on the coating 3 is stepped by an etched-out annular groove 6. The depth to which the annular groove 6 is etched is such that only a very thin bridge portion 7 remains relative to the edge zone 5, and defines the centre 8 of a diaphragm formed from the plate 4.This centre surface 8 is not as thick as the edge zones 5 and, as a result of the inner bridge portions 7, can be deflected towards the carrier 1, and reduce the distance remaining relative to the carrier, to an extent which increases with an increase in the pressure p which is applied to the plate 4 and which is to be measured by the measuring device.

In order to measure the pressure p, and the reduction in the distance between the centre surface 8 and the carrier 1 caused thereby, by the eddy current measuring method, a coil 10 is located on the top side 2 of the carrier 1 opposite the centre surface 8 and comprises a plurality of spiral convolutions and has terminals 11 and 1 2 which are lead rearwardly out of the carrier 1 in an air-tight manner.

In detail, the diameter of the conductor of the coil 10 is chosen to be very small. In a preferred embodiment, the coil 10 comprises a very narrow conductor which is only approximately 0.05 mm thick and which is applied either by vapour deposition or printing or is etched out by the etching method known in connection with thinlayer technology with the use of photo-lacquer from a conductive coating of copper or the like which is applied coherently.

When an alternating current of sufficiently high frequency flows through the coil 10, the magnetic field produced by the coil induces in the centre surface 8 of the plate 4, acting like a diaphragm, eddy currents whose strength increases according to the extent to which the centre surface is deflected towards the coil 1 0. These eddy currents result in a substantial decrease in the impedance of the coil 10.

In order to measure the variation in the impedance of the coil 10 and to be able to detect the pressure p acting upon the plate 4, and RC multivibrator 20 is provided in the evaluation circuit illustrated in Figure 2 and supplies, as an output voltage, a square-wave oscillation of the kind which is indicated at 21 and which has a frequency fof approximately 200 KHz with an amplitude of 5 V. The coil 10 is connected to the square-wave voltage generator 20 by way of a series resistor R 1 and, the shorter the distance between the coil 10 and the diaphragm member indicated at 9, the greater is the change in the impedance value of the coil 10.The characteristic of the square-wave voltage U plotted against time t is shown in the uppermost curve of Figure 3, while the curve located therebelow shows the characteristic of the voltage drop U, which appears across the coil 10 beyond the resistor R1.

This voltage drop U1 is shown in the second curve of Fig. 3 and is used as a measuring signal for which purpose it is rectifier by means of a diode D and is stored in a capacitor Cl. The value of a resistor R2 connected in parallel with the capacitor C1 essentiaily determines the discharge rate of the quantity of electricity stored in the capacitor C1 and thus, together with a low-pass filter R3 and C2 connected on the output side thereof, determines the limiting frequency of the evaluation circuit. The voltage Uc on the capacitor C1 is shown in the third graph of Fig. 3.

Thus, a direct output voltage Ua (Fig. 2 and the lowermost graph of Fig. 3) appears at the evaluation circuit and its value increases as the distance between the diaphragm plate 4 or the centre surface 8 thereof and the coil 10 increases.

One particular advantage of the illustrated measuring device and evaluation circuit resides in the fact that the output voltage Ua varies substantially linearly by approxlmately 1 80 ml when this distance is varied between 0 and 140 ,um. these values being obtainable with a sensor coil 10 having a diameter of 12 mm.

Alternatively, other changes in distance can be detected by the eddy current method by means of a coil 10 in the same manner as in the abovedescribed embodiment provided as a pressure sensor and can be converted to an electrical control or regulating quantity by means of the evaluation circuit specified, it only having to be ensured that eddy currents can be induced in a surface whose distance from the coil is variable.

In the embodiment illustrated in Figure 1, it is possible that difficulties may arise in securing the metal plate, serving as a diaphragm, to the carrier 1 in a pressure-tight manner, particularly when the carrier 1 is made from ceramic material.

Figure 4 shows an embodiment in which the carrier 1 is secured to the inside of the base of a pressure-stable cup 23 of U-shaped crosssection. The edge zone 5 of the plate 4, serving as a diaphragm, is applied to the front rim 24 of the cup 23 and is connected to the cup by means of a soldered or welded joint 25.

In the embodiment of Figure 5, in contrast to Figure 4, the carrier 1 of the coil 10 is not accommodated on the bottom of the cup 23 but is accommodated in an open rebate 27 which is located in the vicinity of the rim 25 of the cup 23 and opposite which is located a rebate 28 of approximately the same depth in the rim 5 of the plate 4 to form the second clamping surface for the carrier 1. Thus, in the same manner as in the embodiment described previously, a mutual pressure-tight seal between the two metal parts 4 and 23 can be achieved by means of a soldered or welded joint 25.

The carrier 1 is of symmetrical construction in the embodiment of Figure 6, in that the carrier 1 also carries a coil 30 on its rear facing the base of the cup 23, and a continuous, etched-out groove 36 forms the base 31 of the cup 23 into a diaphragm which is displaceable relative to the carrier 1 in dependence upon pressure. An intensified measuring signal, which can be evaluated in the evaluation circuit of Figure 2, is produced in the arrangement of Figure 6 which is sealed in a pressure-tight manner by a welded or soldered joint 25 at the edge between the two metal plates 4 and 23 in the same manner as in the embodiment described previously.

The use of a square-wave alternating voltage has the following advantages: a) Substantially simpler generation of the carrier signal by a multivibrator, b) the frequency of the carrier signal does not affect the sensitivity of the measuring device, or affects it to only a negligible extent, over a wide range (such as from 10 KHz to 1 MHz), c) sensors which, owing to their low inductance, would have to be fed with a very high-frequency, sinusoidal alternating voltage in order to produce an evaluable sensitivity, can be operated with a relatively very low-frequency, square-wave carrier signal.

Claims

1. A measuring device for the contactless detection of the distance between a metal surface and a counter-surface, in which a spiral electrical coil is disposed on a counter-surface which faces the said metal surface.

2. A measuring device as claimed in claim 1, in which said counter-surface is planar.

3. A measuring device as claimed in claim 1 or 2, in which the counter-surface is on a nonmetallic carrier.

4. A measuring device as claimed in claim 3, n which the carrier is made from insulating materirsi.

5. A measuring device as claimed in claim 4, in which the coil is applied to the carrier by vapour deposition or printing.

6. A measuring device as claimed in any of claims 1 to 5, in combination with an evaluating circuit which comprises a generator for generating an oscillatory voltage of approximately 100 KHz to 10 MHz, for feeding the coil, a series resistor by which the coil is connected to the generator, and means for detecting the voltage peaks which appear across the coil and which vary in dependence upon the distance to be detected.

7. A measuring device as claimed in claim 6, in which the generator frequency is approximately 200 KHz.

8. A measuring device as claimed in claim 6 or 7, in which said means comprises a rectifier connected to the coil and a filter network connected to the rectifier and comprising at least one storage capacitor disposed in a shunt arm and a discharge resistor connected in parallel with the storage capacitor.

S. A measuring device as claimed in claim 8, in which said rectifier comprises a diode.

10. A measuring device as claimed in claim 8 or 9, in which the filter network further comprises a series resistor connected to the discharge resistor and to the rectifier so as to be in series with the latter, and a shunt capacitor connected to the series resistor.

1 A measuring device as claimed in claim 8, 9 or 10, in which the generator comprises an astable multivibrator.

12. A measuring device as claimed in any preceding claim, which is a pressure transducer and in which said metal surface is on a pressure responsive diaphragm.

13. A measuring device as claimed in claim 12, in which the carrier is disposed on the inside of the base of a cup.

14. A measuring device as claimed in claim 13, in which the cup is made from metal.

1 5. A measuring device as claimed in claim 12, which includes a metal cup having in the vicinity of its rim an open rebate in which the edge zone of the carrier is received.

1 6. A measuring device as claimed in claim 15, in which the plate has a rebate in which the edge zone of the carrier is received.

17. A measuring device as claimed in any of claims 14 to 16, in which a continuous groove is etched into the base of the metal cup, whereby the central portion of the cup base acts as a diaphragm, a second spiral coil being provided on the carrier opposite said central portion.

1 8. A measuring device as claimed in any of claims 14 to 17, in which the rim of the cup which abuts against the plate is sealed to the latter by a welded or soldered joint.

1 9. A measuring device, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the drawings.